Powder coating compositions (dry coatings which do not contain water or organic solvents) can be produced by (1) dry blending or mixing a thermosetting base resin, a curing agent (also referred to as a cross-linker), and other additives which may include, but are not limited to, pigments, adhesion modifiers, flow modifiers, degassing agents, ultraviolet light stabilizers or absorbers, slip agents, catalysts, anti-mar agents, etc., (2) melting and kneading the mixture, as for example in a twin-screw extruder, under conditions such that cross-linking of the base resin does not occur, (3) grinding or pulverizing the extrudate to produce a powder, and (4) classifying the powder on the basis of size to produce the powder coating composition. A powder coating composition so made can then be applied to a substrate such as an electrical appliance, automotive part, etc., by electrostatic spray methods or fluidized bed dipping processes and then baked to give a coating film. The various ingredients which make up the dry blend may also be ground or pulverized before dry blending or mixing to improve the blending or mixing process.
EP 678564 discloses a formulated powder coating composition comprising particles of a binder resin and a hardener, which particles have a specified size and an external additive on their surface.
WO 98/36011 discloses polyester particles having mean particle size less than 50 micrometers. The polyester particles are taught to be uniformly colored and spherical and have a monomodal particle size distribution where d90-d10/d50 is less than or equal to 2.5. The particles can be melted at less than 200xc2x0 C. to form a continuous coating.
U.S. Pat. No. 5,708,039 discloses a formulated powder coating composition of generally spherical particles and of a specified particle size produced by dissolving the ingredients making up the particles in a supercritical fluid and spraying, wherein about 96% by volume of the powder coating particles have a size about 20 micrometers or less.
U.S. Pat. No. 5,498,479 discloses powder coating composition with specified particle size requirements, as well as an external additive attached to the surface of the particles.
EP 687714 discloses a formulated powder coating composition in which the average particle diameter is 20 to 50 micrometers and standard deviation of the particle size distribution is not greater than 20 micrometers. The powder coating is sized by grinding and then classifying.
U.S. Pat. No. 5,468,813 discloses a powder coating composition in which particles of a curing agent have an average particle size of 0.3 to 7 micrometers. Curing agent particles having a particle size not smaller than 20 micrometers account for not more than 10% by weight of all the curing agent particles. The curing agent characteristics are attained by adjusting the particle size by grinding of the curing agent. The patent discloses that when the average particle size is greater than 7 micrometers, the state of mixing of the binder resin and curing agent becomes poor, and coating films with good smoothness can no longer be obtained.
U.S. Pat. No. 5,455,290 discloses a thermosetting epoxy resin-based powder composition which contains a high melting polycarboxylic anhydride in the form of fine particles 5 to 149 micrometers in average size with calcium silicate as a filler. The anhydride cross-linker has a specified size, but the particle size distribution is broad, the particles are not spherical. The patent does not teach how to prepare the particles.
WO 92/00342 discloses a process for making a thermosetting powder coating composition comprising forming a molten mixture of polymer, a curing agent, and optional coloring agent, atomizing the melt into droplets and cooling to form solid powder particles. The patent application discloses narrow particle size distribution with spherical particles prepared by rotary atomization, 2-fluid atomization, etc.
General discussions of spray-drying, melt spraying (prilling), and atomization can be found in the following: Size Reduction and Size Enlargement, Chapter 20, 20-80 and 20-81, R. L. Snow, T. Allen, B. J. Ennis, J. D. Litster, Perry""s Chemical Engineers Handbook, Seventh Edition, R. H. Perry, D. W. Green, J. O. Maloney, Eds., McGraw-Hill, N.Y., 1997 and references contain therein.
Common glycidyl methacrylate (GMA) acrylic powder coating compositions contain a glycidyl-functional cross-linkable base resin, a curing agent which is an acid-functional cross-linker containing at least two carboxylic acid groups, as well as one or more of the additives described above. Common acid functional cross-linkers include, among others, dodecanedioic acid (DDDA) and its anhydride.
DDDA flakes are produced commercially by use of a chilled roll flaker, in which molten DDDA is coated on a chilled, rotating drum and scraped off therefrom with a blade. Abrasion of the flaker blade can result in small fibers and blade particles being trapped in the resulting DDDA flakes. The use of such flakes to make a powder coating composition can result in unacceptably rough and marred coating finishes, a particular problem for automotive clearcoat applications.
Flakes of DDDA (and crystals of other curing agents) are sometimes ground or milled into smaller particles which are often angular, jagged and irregular, and thus mix or blend inefficiently with a base resin during the dry blending or mixing step in preparing a powder coating composition. Such particles also disperse inefficiently during the extrusion step in preparing a powder coating composition, resulting in inefficient use of energy to operate the extruder, increased cycle times for the extruder and nonuniformity in the extrudate prior to grinding it.
Often the curing agent and each of the additives which are used to make a coating composition are separately ground before being dry mixed or blended with each other and the base resin to make the powder coating composition. Such compositions generally cannot contain liquid additives and generally produce final coatings which have limited flow-out, smoothness, gloss and xe2x80x9cdistinctness of imagexe2x80x9d (DOI) .
There is a need in the art for curing agent particles which are not made using a flaker, which are not angular, jagged or irregular, which can be mixed or blended efficiently during the dry blending or mixing step of powder coating composition production, which can disperse efficiently during the extrusion step of powder coating composition production, which impart good flow-out, smoothness, gloss and DOI to coatings, which allow for incorporation of liquid additives, and which allow for the production of masterbatches of single particles which contain both cross-linking agent and additives. The present invention satisfies these needs.
In its broadest aspect, the present invention is a curing agent powder comprising substantially spherical particles of at least one compound selected from the group consisting of azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, brassylic acid, and the anhydrides of said acids, said particles having a particle size distribution as follows:             d90      -      d10        d50     less than   1.90
wherein:
d50 is a particle diameter at which 50% of the particles have diameters which are greater or smaller than the d50 value;
d90 is a particle diameter at which 90% of the particles have diameters which are smaller than the d90 value;
d10 is the particle diameter at which 10% of the particles have diameters which are smaller than the d10 value; and
d50 is 8 to 30 micrometers, with 10 to 25 micrometers being preferred and 10 to 20 micrometers being most preferred.
In a second aspect, the present invention is a powder coating composition comprising the curing agent powder, above, and a cross-linkable base resin.
In a third aspect, the present invention is a process for making a curing agent powder comprising spraying a molten curing agent comprising at least one compound selected from the group consisting of azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, brassylic acid, and the anhydrides of said acids, from a nozzle into a walled chamber to form a spray comprising droplets of said curing agent, said chamber being sized to allow said droplets to solidify before they contact the wall of said chamber.
Because the spherical particles of the present invention are not made using a flaker, they are not subject to contamination with flaker blade material.
The spherical particles of the present invention mix or blend more efficiently during the dry blending step in the production of powder coating compositions than particles produced by grinding or jet milling which are angular, jagged, and irregular.
The spherical particles of the present invention disperse more efficiently than irregular particles during the extrusion step in the production of powder coating compositions, which reduces the energy required for operation of the extruder, reduces the cycle time of the extrusion, and gives a more uniform extrudate prior to grinding.
Powder coating compositions which contain the spherical particles of the present invention give, after baking and cooling, final coatings with better flow-out, better smoothness, better gloss, and better distinctness of image (DOI) than can be obtained from otherwise comparable compositions made using irregular particles.
The present invention allows the incorporation of liquid materials into the coating compositions without the need for separate carrier particles coated with the liquid materials.
The present invention allows the production of so-called masterbatches of single particles containing both curing agent and additives, thereby eliminating the need to grind separately the curing agent and each of the additives, thereby resulting in energy and cost savings in the manufacturing process used to make powder coating compositions.